The present invention relates to a method and apparatus for thermal chemical reactions. The method and apparatus can provide an enhanced reaction rates for thermal chemical reactions.
Thermal chemical reactions are those chemical reactions that produce (exothermic) or consume (endothermic) heat. Examples of thermal chemical reactions include hydrocarbon conversion reactions such as steam reforming, water-gas shift reactions and combustion. These well-known reactions are usually carried out in the presence of a catalyst at temperatures up to about 1300xc2x0 C. Because the intrinsic kinetics of a thermal chemical reaction can be much faster than the heat transfer rate between the reaction vessel and the thermal sink, source or environment, the actual rate of product production (i.e., the observed rate) is slower than the intrinsic rate. Intrinsic kinetics means the rate at which products could theoretically be formed at the catalyst surface.
Limited production rates may result from longer residence time which is typically seconds to minutes in conventional thermal chemical reaction vessels. As it is conventionally defined, residence time is equal to the volume of the reaction zone divided by the inlet volumetric flow rate of reactants at the reaction system""s temperature and pressure. The reaction zone is the total volume of the catalyst and surrounding area through which reactants and products flow.
An example of these limited production rates can be seen in the water gas shift reaction which is conventionally carried out in fixed bed reactors. In the water gas shift reaction, carbon monoxide and water are converted to carbon dioxide and hydrogen. Conventionally, this reaction suffers from multiple-second residence times (a kinetic impediment) when carried out in fixed bed reactors. Theoretical kinetics suggests that residence times on the order of milliseconds could, theoretically, be obtained. There are two kinetic retarding aspects to conventional reactors. The first is a diffusion limitation as reactants diffuse into and out of a catalyst-bearing porous pellet and the second is a heat transfer limitation which is a combination of heat transfer parameters (thermal conductivity and length) of catalyst supports and overall reactor geometry (shape, size, and distance to the external heat exchanger). Because the water gas shift reaction is critical to a multi-reactor fuel processing system that supports distributed energy production through the use of a fuel cell, there is a need for a smaller, faster water gas shift reactor.
Another example of a thermal chemical reaction is in the conventional methane steam reforming reactor which produces synthesis gas at an average residence time of several seconds and with an effectiveness factor of 0.01 to 0.05 as reported by Adris, A., Pruden, B., Lim, C., J. Grace, 1996, xe2x80x9cOn the reported attempts to radically improve the performance of the steam methane reforming reactor,xe2x80x9d Canadian Journal of Chemical Engineering, 74, 177-186. In a typical industrial operation, the methane to steam ratio is run at 3:1 to prevent coke formation. Efforts to improve heat transfer between the reaction vessel for this endothermic reaction and the thermal source have made only modest improvements in product production rate.
Thermal reactions have long been, and continue to be, conducted in huge volumes on production scales requiring very large capital investments, typically greater than $100 million. Not surprisingly, there have been extensive efforts, over a long period of time, aimed at improving the speed and efficiency of these reactions. Despite these attempts, there remains a need for a method and apparatus that increase the heat transfer rate between the reaction vessel and the thermal sink or source and thereby approach the theoretical intrinsic kinetic rate of reaction and production.
The present invention provides methods and apparatuses for obtaining an enhanced production rate per reaction chamber volume of a reaction chamber with an inlet and an outlet for a thermal chemical reaction, wherein a ratio of the enhanced production rate per reaction chamber volume to a conventional production rate per conventional reaction chamber volume for the thermal chemical reaction is at least 2. For example, for conventional steam reforming, residence time is on the order of seconds whereas with the present invention, residence time is less by a factor of 2, on the order of milliseconds to tens or hundreds of milliseconds. In one aspect, the invention includes:
(a) a porous insert within the reaction chamber volume, wherein a reactant flow substantially completely passes through the porous insert wherein the reaction chamber volume with the porous insert has a mean porosity less than 1 and a mass transport distance of reactants to a catalyst site of no greater than 3 mm;
(b), the reaction chamber volume with a length parallel to a bulk reactant flow, the length less than or equal to 6 inches, and with a height (a thermal distance from the heat sink to the heat source) less than or equal to 2 inches, thereby transferring reaction heat at an enhanced heat transfer rate through the porous insert; and
(c) a heat transfer chamber in thermal contact with the, reaction chamber volume, serving as a heat sink or heat source, the heat transfer chamber transferring heat at said enhanced heat transfer rate across a wall between the heat transfer chamber and the reaction chamber, thereby obtaining the enhanced production rate per reaction chamber volume for the thermal chemical reaction wherein a ratio of the enhanced production rate per reaction chamber volume to a conventional production rate per conventional reaction chamber volume for the thermal chemical reaction is at least 2.
These features have been found to cooperate with the reaction kinetics in terms of transferring heat at a rate sufficient to avoid substantial impediment of the kinetics. These features are effective for both catalytic and non-catalytic thermal chemical reactions. For catalytic chemical reactions, addition of a thin catalyst layer ( less than 150 microns, xcexcm, more preferably less than 50 xcexcm) upon the porous insert substantially reduces the diffusion pathways of reactants to catalyst sites compared with more severe limitations of reactant diffusion within ceramic pellets ( greater than 1 mm) as in conventional systems. Thus, according to the present invention, for catalytic thermal chemical reactions, both kinetic impediments are substantially reduced permitting realization of theoretical or near theoretical reaction kinetics. More specifically, a water gas shift reactor made according to the present invention has {fraction (1/10)}th to {fraction (1/100)}th the size of conventional processing hardware for the same production output.
The present invention further provides a method and apparatus (vessel) for providing a heat transfer rate from a reaction chamber through a wall to a heat transfer chamber (exothermic reaction) or providing heat from a heat transfer chamber through a wall to a reaction chamber (endothermic reaction) substantially matching a local heat transfer requirement of a catalytic thermal chemical reaction. An important aspect of this invention is the thermal distance defined on a cross sectional plane through the vessel inclusive of a heat transfer chamber, reaction chamber and a wall between the chambers. The cross sectional plane is perpendicular to a bulk flow direction of the reactant stream, and the thermal distance is a distance between a coolest position and a hottest position on the cross sectional plane. The thermal distance is of a length wherein the heat transfer rate from (or to) the reaction chamber to (or from) a heat transfer chamber (heat exchanger) substantially matches the local heat transfer rate.
The invention includes a process for the catalytic conversion of at least one reactant in a thermal chemical reaction, in which at least one reactant is passed into at least one reaction chamber; heat is transferred to or from the reaction chamber into at least one heat exchanger; and at least one product is obtained. The reaction chamber contains a catalyst that catalyzes; the reaction of the reactant or reactants. In preferred embodiments, the process has one or more of the following characteristics: at steady state, at least 0.6 W/(cc of total reactor volume) of heat is transferred, where total reactor volume is defined as the sum of the volume of the reaction chamber(s) and heat exchanger chamber(s) including the volume of chamber walls; the contact time of the reactant with the catalyst is less than about 0.3 seconds; and the pressure drop through the reaction chamber is less than about 15 psig.
One example of a thermal chemical reaction that can be conducted using methods and reactors of the present invention is steam reforming of a hydrocarbon. In this process a feed stream comprising hydrocarbon gas and steam is passed into a reaction chamber which contains a catalyst that catalyzes the reaction of hydrocarbon gas and steam to produce a gaseous mixture comprising at least carbon monoxide and hydrogen gas. This process can produce more than 0.01 SLPM of hydrogen gas per cubic centimeter of total reactor volume.
The present invention also provides a reactor for the catalytic conversion of at least one reactant in a thermal chemical reaction, comprising: at least one reaction chamber containing a porous catalyst insert; and at least one heat exchanger that is in thermal contact with the reaction chamber. The reaction chamber has a length less than or equal to 6 inches and a height less than or equal to 2 inches. The porous catalyst insert comprises a porous metal foam having open cells ranging from about 20 ppi to about 3000 ppi.
The invention also includes a reactor in which the reaction chamber has a height less than or equal to 2 inches; and wherein at least one heat exchanger and at least one reaction chamber are configured such that, during steady-state operation, at least 0.6 W of heat per cc of total reactor volume can be transferred between the heat exchanger and the reaction chamber.
The invention also includes a process for the catalytic conversion of at least one reactant in a thermal chemical reaction in which at least one reactant is passed into at least one reaction chamber that contains a catalyst that catalyzes the reaction of the at least one reactant; transferring heat to or from said at least one reaction chamber from or into said at least one heat exchanger; and obtaining at least one product from the reaction chamber; where the step of transferring heat, at steady-state, transfers at least 0.6 W of heat per cc of total reactor volume, such that, at steady state, the catalyst is maintained within a temperature range that reduces the formation of at least one undesirable chemical reaction product. Alternatively, the formation of undesirable chemical product(s) can be reduced by utilizing a contact time of less than about 0.3 seconds, thereby suppressing slow reactions that may form an undesirable chemical reaction product. Undesired chemical products can result from secondary reactions or slow parallel reactions. In the water-gas shift reaction, desirable products include carbon dioxide and water, and an undesirable product is methane. In steam reforming of a hydrocarbon, desirable products include hydrogen and carbon monoxide and/or carbon dioxide, and an undesirable product is coke.